The present invention relates to devices for separating oxygen from a more complex gas containing oxygen to deliver the separated oxygen for use, and more particularly, the invention relates to solid state electrochemical devices for separating oxygen from a more complex gas, and even more particularly to a pressure driven ceramic oxygen generating system with integrated manifold and tubes.
In principle, there are several types of oxygen concentration systems: adsorption, polymer membrane, electrolysis, chemical processing, cryogenic processing and ceramic oxygen generation. Each of these systems has a unique combination of flow rate, oxygen purity, pressure, and use pattern. Ceramic Oxygen Generating Systems (COGS) are unique in that they provide extremely high purity oxygen due to the fact that the ceramic materials used in the systems allow only oxygen to pass through the material.
The present invention takes advantage of the phenomenon that, when raised to sufficient temperature (over 600° C.), certain ceramic materials can easily have oxygen ions dislodged from within the ceramic's lattice structure. That is, when these materials are formed into a thin membrane, a driving force and a fresh oxygen supply on one side of the membrane induce a flow of oxygen through the membrane. Since the oxygen ions that migrate through the membrane are actually elemental constituents of the ceramic lattice, only oxygen can flow through the membrane. Other gaseous species, such as nitrogen or argon, cannot migrate through the lattice as these species are incompatible with the ceramic lattice structure. As a result, the derived concentrated product gas stream contains high purity oxygen gas.
Oxygen flow across a ceramic membrane is commonly induced through two methods: electrically driven or pressure driven. In an electrically driven system, an electric potential is induced across the ceramic membrane using special coatings and a voltage supply. The oxygen ions present within the ceramic when at high temperature (over 600° C.) are driven through the membrane by the voltage difference between opposing surfaces of the membrane. Since the oxygen ions are charged species, the number of ions migrating through the membrane is directly related to the magnitude of the electrical current passing through the membrane where higher currents result in more ion migration.
In a pressure driven system, the ceramic is comprised of a mixed conductor composition which can conduct both oxygen ions and electrons. An inlet compressed air source and/or a vacuum outlet at the product side generate a pressure differential across the ceramic membrane. Under elevated temperature, the ceramic membrane ionizes oxygen molecules on the membrane surface exposed to gases with higher oxygen partial pressure. Oxygen ions then diffuse across the ceramic membrane and recombine into oxygen molecules at the opposite surface which has lower oxygen partial pressure. As discussed above, only oxygen ions are compatible with the ceramic lattice such that only oxygen gas is generated at the product outlet, thereby producing an extremely high purity oxygen gas product.
One approach to producing ceramic membrane materials for use with COGS systems is the Integrated Manifold and Tubes (IMAT) module design. For example, U.S. Pat. Nos. 5,871,624; 5,985,113; 6,352,624; 6,685,235; and 6,783,646, each owned by the assignee and incorporated in their entireties herein, teach IMAT designs for use in an oxygen generating system. However, such prior IMAT designs are limited to electrically driven COGS systems. As required by such electrically driven systems, the prior MINT designs include the provisions of a number of additional conductive coatings and methods of producing parallel and serial electrical connections between the individual tubes within the modular array. The additional coatings and electrical connections increase the complexity of the system and time required to produce a complete module. Further, any surface area other than that coated by the conductive coating is unavailable for generating oxygen, thus leading to decreased utilization potential of the ceramic membrane materials. Still further, to achieve the desired oxygen generating performance, the size and thickness of each tube within the array needs to be closely matched (if not exactly the same) so that there would be relatively uniform distribution of voltages for the components connected in serial relation. This requires tight design tolerances and vigilant quality control. Yet still further, due to the relatively large magnitudes of electrical current passing through the IMAT, self-heating of the ceramic is significant and requires careful thermal management to optimize oxygen production without damaging or destroying the IMAT module.
It is therefore an object of this invention to provide a ceramic oxygen generator for a pressure driven COGS system thereby obviating the need for conductive coatings and associated electrical connections.
Another object of this invention is to provide a ceramic oxygen generator for a pressure driven COGS system wherein the ceramic material comprises a mixed conducting material.
A further object of this invention is to provide a ceramic oxygen generator for a pressure driven COGS system wherein a manifold structure for receiving the separated oxygen is an integral part of the manufactured generator structure and is less costly to make.
Still another object or this invention is to provide a ceramic oxygen generator for a pressure driven COGS system which is of a modular configuration and thereby provides a simple “building block” approach to meet differing requirements for amounts of oxygen to be generated.